1. Field of the Invention
The present invention relates generally to the field of systems for measuring pollutants in a gas stream. More specifically, the present invention discloses a sample conditioning system modification for removing selenium to enable more accurate measurements of mercury in the gas stream.
2. Statement of the Problem
Selenium is a trace constituent present in the flue gas from some combustion sources, such as coal-fired electric power plants. Selenium is typically present as elemental selenium or selenium dioxide (SeO2). The presence of selenium or selenium compounds can interfere with the accurate measurement of other pollutants (e.g., mercury) in the gas stream.
Mercury is also a trace constituent present in some flue gas streams. The mercury is typically present as elemental mercury or in an oxidized state (e.g., HgCl2 or HgO). Prior to measuring the mercury concentration, the sampled gas stream is first processed in some conventional manner to chemically reduce all of the oxidized mercury to elemental mercury. For example, one approach has been to add propane or other reducing agents to the flue gas stream, which consume available oxygen and other oxidizing agents. The result is a reductive environment capable of catalytically reducing chemical compounds found in the gas stream. The mercury concentration in this processed gas stream is then analyzed using cold vapor atomic absorbance (CVAA) or other conventional techniques that are specific for elemental mercury.
It has been observed that at some sites, the tubing downstream from the catalyst quickly becomes fouled and removes mercury from the gas stream. The distinct garlic smell of hydrogen selenide (H2Se) can be detected in the fouled tubing. Laboratory testing confirms that hydrogen selenide can react with elemental mercury to form mercuric selenide, which will not be detected by the CVAA detector.
Thus, a problem arises in the reduction step in that at least a portion of the selenium compounds in the gas steam will also be reduced to hydrogen selenide. Hydrogen selenide can react with mercury to form mercuric selenide (HgSe), thus interfering with the mercury measurement.
Solution to the Problem. The present invention seeks to eliminate this interference by scrubbing the gas exiting the mercury reduction unit with a basic aqueous solution to remove hydrogen selenide from the processed gas stream. Hydrogen selenide is a weak acid that is less soluble in acidic solutions. In particular, acid gases (e.g., HCl) present in the flue gas can lower the pH of the scrubbing solution to a point where the hydrogen selenide is no longer soluble to a significant degree.
To improve the solubility of hydrogen selenide, ammonia or another basic reagent can be injected into the gas stream prior to the condenser to increase the pH of the scrubbing solution and improve the removal of hydrogen selenide.